An extremely large range of methods and apparatus are presently used for the treatment of water. These are based on various principles. The principles include micro-filtration and aerobic breakdown of waste material. Both principles are often combined. Furthermore, one or both of the principles are used together with a disinfecting arrangement to achieve water that is suitable for discharge into the environment and even potable in some cases.
A problem with presently available systems is the level of maintenance required and the cost of supply and installation. Micro-filtration systems require regular cleaning of micro-filters. Furthermore, micro-filter elements can be expensive. Systems based only on aerobic breakdown of waste material, particularly black water, often do not achieve sufficiently low levels of faecal matter and other effluent components such as eggs of intestinal worms.
A popular form of filtration is the use of sand filters. Sand filters are attractive since they are relatively cheap to install and, with an appropriate selection of sand, can provide a filtrate which is suitable for discharge into the environment.
This invention is directed to a system that uses slow sand filtration. Slow sand filtration has been used successfully in Europe since the early 1900's, and is still a popular method of treating municipal waste water. Research has shown that slow sand filtration can effectively remove Giardia cysts and coliform bacteria from raw water. Slow sand filtration has proved to be a cost-effective and low maintenance treatment process.
In general, a slow sand filter comprises a bed of sand which is supported by a layer of gravel. The filtration process is effective in removing solids, precipitates, turbidity and in some cases bacterial particles that produce bad taste and odour. The effectiveness of the slow sand filter lies in the provision of a very high surface area by the particles of sand which is colonised by bacteria that break down harmful pathogens in the waste water. The efficiency of the process is dependent on the particle size distribution of the sand, the ratio of surface area of the filter to depth and the flow rate of water through the filter.
It has been found by the Applicant that the selection of suitable sand is extremely difficult. Furthermore, in order to inhibit blocking, it is necessary to interpose one or more layers of a suitable aggregate between layers of the sand.
The sand used should be as insoluble in water as possible. In the event that the sand is soluble, the filtrate can leach out undesirable chemicals. For example, with alluvial sands, chemicals such as sulphates, which cause acidity in water, can leach out.
Applicant has conceived the present invention to address the problems associated with presently available sand filters. Furthermore, Applicant has developed a filtration apparatus that has an improved configuration.
Applicant has found that sand filtration can be successfully used for filtering circulating, warm water that can harbour pathogens. An example of such circulating water is the cooling water used to cool air conditioning units such as those used to cool multi-storey apartment buildings. Accordingly, Applicant has conceived an aspect of the invention to provide an effective treatment of such circulating water based on the principles of slow sand filtration.